The present invention relates generally to vacuum deposition systems and more particularly to a chemical vapor deposition method used in the fabrication of integrated circuit devices.
It is well known in fabricating integrated circuit devices to employ vapor phase chemical processes to form layers of various materials on a substrate. For example, one method of forming silicon dioxide on a substrate is a chemical vapor deposition process involving the pyrolysis of tetraethylorthosilicate (TEOS). The TEOS is typically vaporized in a heated reservoir and the vapor is transported to a reaction chamber, which contains the substrates, by an inert carrier gas. This method is disclosed, for example, in U.S. Pat. Nos. 3,243,314 to Lehman et al., assigned to the assignee of the present invention, and 3,556,841 to Iwasa et al.
It is important to precisely control the rate of pyrolysis of TEOS in the reaction chamber in order to form a silicon dioxide layer having the desired thickness, uniformity and chemical and electrical characteristics. This, in turn, requires a controlled mass flow or transfer of TEOS from the reservoir to the reaction chamber. In one known method for metering the mass flow rate the temperature of the reservoir containing the liquid to be vaporized is adjusted to provide a constant flux of vapor. Another method, disclosed in U.S. Pat. Nos. 4,393,013 and 4,436,674 to McMenamin, controls the rate that a carrier gas is bubbled through the reservoir to provide a constant flow rate of vaporized fluid, based upon the difference between the actual pressure in the reservoir and a reference pressure.
The system described in U.S. Pat. No. 4,341,107 to Blair et al. employs thermal conductivity type mass flow sensors at the inlet and outlet of a reservoir. The sensor measurements are compared to provide a signal, representing the mass flow rate of the vaporized liquid, which is used to control the flow rate of the carrier gas into the reservoir.
A reduced pressure chemical vapor deposition system is disclosed in U.S. Pat. No. 3,934,060 to Burt et al. This system uses a vacuum to pull the vaporized source material from a reservoir to a reactor. A predeposition vacuum process is employed to remove impurities first from the TEOS source and then from the wafer carrier and wafers in the diffusion tube or reactor.
Systems which adjust the temperature of the reservoir to control the mass flow rate of the vapor to the reactor have an inherently slow response time and thus are prone to wide variations in the thickness and uniformity of the deposited film. In other systems the reservoir geometry and liquid level are critical, and the effects of variations in bubble size are difficult to control in carrier gas type systems. In addition, it is desirable to increase the flow rate and pressure to provide a higher deposition rate while maintaining repeatability from batch to batch and uniformity in thickness and density of the deposited material.